1. We have isolated and characterized a developmentally regulated eucaryotic gene, the gene for type I alpha2 collagen. Its coding information is subdivided in at least 52 exons. The size of the exons, which is remarkably conserved, indicates how the gene was assembled during evolution by multiple duplications of a single genetic unit. 2. We have characterized the promoter of this gene by DNA sequence. The region before the start site contains three large overlapping dyads of symmetry which could constitute binding sites for regulatory proteins. 3. We have constructed recombinant plasmids in which the collagen promoter is placed adjacent to a small bacterial gene, the gene encoding chloramphenicol acetylase, to study the expression of this recombinant in animal cells. 4. We have begun to isolate one of the human collagen genes (alpha1 type I) and two other chick collagen genes. 5. We have shown that the two promoters of the galactose operon of E. coli are functional in intact cells and have obtained additional promoter mutants in the E. coli galactose operon which elucidate the complex regulatory mechanisms of this operon.